The invention relates to a method and a system for producing a multistratum, preset, i.e., preconsolidated, thread- or fiber fabric that is used as an elongated semi-finished product for reinforcing parts made of plastic or resins and that in the individual strata comprises thread- or fiber sheets, each of which is oriented differently, whereby in at least one of the strata—a stationary weft stratum—thread- or fiber sheets is oriented largely parallel to the working direction, whereby at least one of the strata comprises thread- or fiber sheets that are stretched between rows of hooks of conveyor chains on both sides of a laying arrangement for producing the thread fabric, whereby the thread- or fiber fabric is supplied to a warp knitting machine at the speed of the aforesaid transport chains and there—stretched between the transport chains—is preset by means of a system of loops and whereby needle loops and plate loops of the system of loops enclose threads or fibers of all strata of the thread- or fiber fabric.
In the production of multistratum, pre-consolidated thread fabrics that are impregnated or cast-in with resin or plastic in a subsequent process and finally cured in a pre-selected shape into semi-finished products for various final products, the exterior thread- or fiber layer is in many respects determinant for the properties or employment of the semi-finished product.
Especially for band-shaped, i.e., elongated, workpieces, flexural resistance transverse to their longitudinal axis to a great extent determines how they can be used. This is particularly true for aircraft skins and ship hulls. In this case there is the additional challenge of making the semi-finished products light and cost-effective.
Another determining feature is the visual surface of such a structure. It is generally desirable that the surface adapts to the structure of the future product or does not interfere therewith.
Thread fabrics of this type are generally preset on the warp knitting machine using the so-called stitch bonding method. In known warp knitting machines, so-called stationary weft threads that extend across lengthy segments exclusively in the working direction of the warp knitting machine cannot be inserted on the right-hand side of the knitted fabric. The “right-hand” side of the knittted fabric is the side the knitting needles first penetrate. If the illustrations of the knitting machine, e.g., FIG. 1, are rotated 90° counterclockwise, it is seen that the aforementioned side is the “right-hand side.” Insofar as the multi-strata fabric is concerned, the outermost stratum on the right-hand side will be designated the “lowermost” stratum and the outermost stratum on the other side will be designated the “uppermost stratum.”
On such machines, the only thread- or fiber fabrics that can be compressed and preset are those that have diagonal or transversely laid thread- or fiber strata on their lower side, the right-hand side of the knitted fabric. Such thread- or fiber systems can be stretched between conveyor chains and guided in the cast-off plane to the loop formation site of a warp knitting machine with no problem.
For assuring symmetrical stability under load of the reinforcing thread fabric, the fabric structure is generally designed approximately symmetrical. One stratum or a plurality of strata of thread sheets that are inclined at different angles to the working direction is/are added to a center layer of weft threads thereabove and therebelow.
For assuring relatively high flexural resistance of the semi-finished product transverse to the working direction, the exterior stratum is designed with a very small angle—generally 30°—to the working direction of the thread fabric. However, this requires extremely long laying arrangements for producing these fiber fabrics. At an angle of inclination of 30° relative to the working direction and a warp knitting machine with a working width of more than 100 inches, the laying arrangement with three laying apparatus is already more than 10 meters long.
The conveyor chains that stretch and transport the thread fabric are subject to high wear. Laying apparatus with large dimensions are expensive and require constant maintenance.
The following documents define the state of the art in this field:
U.S. Pat. No. 3,761,345 provides originally used systems and methods for producing the cited thread- or fiber fabric. In each case, the lowermost thread layer or stratum is provided with primarily transversely-oriented thread- or fiber sheets that are stretched between the conveyor chains.
Most of the other thread layers are laid cross-wise or in a zigzag pattern so that there are numerous areas on the thread fabric that have different thicknesses.
To avoid the visually unsatisfactory surface, fiber snippings are applied to the left-hand side of the thread fabric and bound thereto in the knitting process. A second knitting process is required for covering the right-hand side of the knitted fabric with the same type of fiber snippings.
In accordance with U.S. Pat. No. 4,325,999, thread- or fiber sheets are placed only parallel to one another to counter areas in the thread sheet that have different thicknesses.
For attaining particularly high rigidity relative to bending transverse to the longitudinal direction, at least two strata that have straight threads parallel to one another and that are inclined only 30° to the working direction were added to the knitted fabric/thread fabric. Such a form for the thread fabric is not suitable either, as already mentioned, for satisfying the user requirements as defined in the foregoing.
With enlarging the working width of the thread fabric to more than 100 inches (=2.52 m) and with the increasing desire for a greater number of thread- or fiber strata, it became increasingly difficult to supply the produced thread fabric in a stable manner between the conveyor chains of a warp knitting machine.
In accordance with DE 198 52 281 A1, positioned below the laying apparatus and between the transport chains is a continuous conveyor belt, the upper run of which was supported from below. However, the dimensions of the conveyor belt made it necessary to reliably guide the thread fabric from the moved conveyor belt across an open distance to the cast-off plane of the warp knitting machine. The diagonal lower thread layer was used for this, as well.
For additionally assuring guidance in the transition from the moved to the frame-fixed guide, in one exemplary embodiment continuous guide wires were used together with the conveyor belt, and these could be guided through the spaces of the needles in the cast-off plane without their being tied up therein. These continuous guide wires could be guided back to the entry of the laying arrangement and onto the conveyor belt. Such guide wires and guiding them are extremely expensive and solve only the last-cited problem.
It can therefore be assumed that the thread fabrics that reinforce large surface area, flat, elongated semi-finished products have a structure such as that illustrated and described in DE 33 04 345 C2 and DE 33 43 048 A1. The required flexural stiffness in one direction is primarily provided by a single stationary thread system or at most by two stationary thread systems.
All other diagonal thread systems contribute only as more or less elastic members in a limited manner to increasing the flexural resistance in the cited direction. For one skilled in the art, it is clearly evident that thread fabrics embodied in this manner are only somewhat stable against bending transverse to the longitudinal direction of the semi-finished product. Generally additional reinforcing elements are added for assuring a predetermined flexural stability, but this increases mass and raises costs.
The object of the present invention is to suggest a method and a system for producing a multistratum preset thread- or fiber fabric that makes it possible to arrange optimally effective reinforcing elements as far as possible from the neutral bending line for assuring the necessary flexural stability of thread- or fiber-reinforced semi-finished products and makes it possible to use to the greatest extent possible existing elements or elements that are low in mass for setting on the thread- or fiber fabric.